The present application relates to computers with built-in speaker systems, and particularly to single-user desktop computers.
Background: Computer-Based Audio
The production of quality sound systems for computers has advanced significantly in recent years. Early personal computers typically had nothing more than a single, small acoustic driver used to produce a beep or series of single-frequency beeps to indicate system status upon startup. Typically, no additional sound producing circuitry was included. However, consumers demanded better audio performance. With recent advances in circuit miniaturization and audio engineering, sound cards capable of excellent sound reproduction are available for desktop computers.
Background: Computer Audio Requirements
In the 1990s, multimedia systems for personal computers have been a very active area of competition between manufacturers. The consumer market strongly demands some sort of sound system, but is also very sensitive to cost.
The normal approach, as of 1998, is to ship a pair of small speakers which are connected to the computer chassis with short speaker cables. A disadvantage of using external speakers is that they force a user to find space on his desktop for them. The present inventors have recognized that it would be highly desirable to provide an economical internal high-quality “default” speaker system onto which additional component speakers can be added if desired.
Background: Acoustical Implications of Computer Chassis Constraints
The consumer-expected structure of small computers (whether desktop, minitower, single-user, and/or personal) puts some constraints on the acoustical characteristics of the computer chassis. These constraints have made it difficult to use the interior volume of the computer chassis as a sealed or ported speaker box.
One constraint is the conventional modularity of personal computers. Many consumers expect to be able to customize with add-in cards for extra functions. Many manufacturers and assemblers also find this flexibility invaluable.
The conventional form factor for a PC chassis, which derives from the IBM personal computers of the 1980s, is difficult to make acoustically sealed. Moreover, because the consumer or assembler can be expected to open the chassis for card insertions, it is possible that one of the covers over the openings for external card connections might be left off; this would drastically change the acoustical characteristics of a sealed chassis.
Another constraint is the airflow requirement for cooling. In configurations with a high-speed microprocessor and maximum hard-drive capacity, airflow through the chassis interior may be required for cooling. It is difficult to accommodate this airflow while acoustically sealing the chassis.
A further constraint is that some mass-storage devices, such as hard drives and CDROM drives, may be very sensitive to vibration. Too much vibration could cause the disk heads to skip or crash. Some earlier computer designs in which a driver was mounted in the front of the chassis were found to cause skipping in CDROM drives.
Background: Spatial Impression
The design of sound reproduction systems is not only based on considerations of electrical/acoustical engineering and physics, but also requires knowledge of psychoacoustics, i.e. how sound is perceived by listeners.
One of the parameters of psychoacoustics is spatial impression. Spatial impression is a term used to in acoustics to define a listener's perception of fullness, width, and impression of being in a three-dimensional space. A listener's perception of the size of a room is influenced by the relationship between direct and indirect (reflected) sound. When a sound is generated in the room, the listener will first hear the sound via the most direct path from the source. Shortly thereafter, the listener will hear the reflections of the sound from surfaces such as walls or ceilings.
Human listeners will assess the size of the space they are in by listening to laterally reflected sound which accompanies a sound signal. A more roomy spatial impression is welcome to many listeners. Thus, in a loudspeaker system it is desirable to have some sound transmission paths which reach the ears of the listener with a certain amount of delay (e.g. 10–60 milliseconds) as compared with the direct transmission path. This delay will give the impression of a spacious listening room by broadening the soundstage, and also by giving the illusion of pushing the sound beyond the physical location of the speakers, even if the listener is listening in a room which is very small.
Another peculiarity of spatial impression is that low frequencies (below 500 Hz) have a disproportionately greater effect on spatial impression than frequencies above 500 Hz. However, it is not easy to produce low frequencies at an acceptable power level with a small speaker system.
To increase spatial impression, the ratio of laterally reflected energy to directly transmitted energy can be increased: the higher this ratio, the greater the spatial impression. An extreme case of this is found in a symphony concert hall, where most sound has been reflected before reaching the listener.
Whether the user is video-conferencing, playing a game, or just working with music in the background, spatial impression plays an important role in the computing experience. That role is growing ever more important as multimedia makes its way further into the business and gaming environments.
Further background regarding spatial impression can be found in: J. Blauert, SPATIAL HEARING (2.ed. 1996); and in M. Barron, “Effects of Early Reflections on Subjective Acoustic Quality in Concert Halls” (thesis, University of Southampton, 1974); both of which are hereby incorporated by reference.
Background: Stereophonic and Surround Sound
Since its introduction in the 1950's, stereo has been regarded as an essential minimum requirement of quality sound reproduction. By splitting the audio signal into right and left channels, stereo tries to simulate a traditional soundstage such as that experienced when one attends a play or concert. However, even stereo has shortcomings when required to accurately simulate a setting where the sound is heard from all around the listener. Stereo's lack of spatiality undermines sonic realism in, for example, a game where aircraft fly overhead from front to back, or footsteps come from off to the side. To heighten sonic realism various “surround sound” schemes have been used to provide at least some speaker output behind the listeners' positions.
Background: Dynamic Speakers
The heart of a loudspeaker is the motor structure that drives the diaphragm or cone. One simple way to generate the motion in a speaker is with a dynamic transducer. This is basically a linear transducer, in which a movable diaphragm (or “cone”) is attached to a coil which is driven by a variable current. The coil is suspended in a constant magnetic field. The current through the coil interacts with the magnetic field to generate a force, which makes the coil and diaphragm oscillate according to the current variations through the coil. Motion of the diaphragm will generate two acoustic outputs: a frontwave output from one side of the moving diaphragm, and a backwave output from the other side.
One fundamental decision in a loudspeaker design is what to do with the backwave so that it will not interfere with the sound of the frontwave. A classic solution is to completely swallow the backwave in the confines of a sealed enclosure; this is known as an “acoustic suspension” speaker. Another popular solution is to route some of the bass backwaves back toward the listening area through a vent. This is known as a “bass reflex” design. Still another method is to use a passive radiator, or a “drone” driver, which passively vibrates with energy from inside the cabinet, not with direct signal from the crossover. All these methods have advantages, especially if the drivers and enclosures are carefully designed.
The response of an assembled loudspeaker is normally quite different from that of an individual driver standing alone (a free-space driver). With a normal free-space driver there will be substantial cancellation at mid and lower frequencies because the back of the driver is approximately the same area as the front of the driver and is radiating at a phase which is exactly 180 degrees out of phase with the acoustic radiation from the front of the speaker. Thus a great deal of engineering effort has been put into designing speakers for optimal loading of the electromechanical driver to produce the desired response. For instance, in an acoustic suspension speaker, the driver is simply mounted in a closed box that provides capacitive loading and damping for the driver's motion. FIG. 4 shows a typical prior art acoustic suspension speaker. A driver 410 consisting essentially of a cone 420 and an electro-acoustic transducer 430 is mounted in a sealed box 440. The walls of the box may be covered with an acoustic damping material 450. The arrangement shown in FIG. 4 is power inefficient because the driver has to push against air in a sealed box, but it does reduce interference between frontside radiation and backside radiation.
Many other speakers use ports and acoustic loading elements which are tuned to include one or more resonances, with the ultimate objective of improving the speaker's power efficiency, linearity, and frequency range. However, a free-space speaker requires different techniques to achieve those objectives.
Background: Wall-Effect
Spatial imaging is a great concern in audio technology. Spatial imaging can often be enhanced by bouncing audio signals off of reflective surfaces toward a listener such that there is a slight time delay between the reflected waves and waves beamed straight at the listener. These reflectors are generally rigid surfaces such as walls (hence the name wall-effect or corner-effect).
In many speaker systems, reflected waves are ignored in favor of the strategy of beaming the audio wave directly at a listener, resulting in a flat sound with no audio depth. Reflected audio waves from a musician's live performance add to the richness of the listening experience but these reflected waves are not present when an audio wave from a recording is beamed directly at the listener. Utilizing the wall-effect allows for a more “real-world” representation of sound because it simulates the reflected waves that would be present from a “live” event.
Background: Baffles
A baffle can be thought of as a plane wall that does not allow sound to pass through it. Some speakers are commonly modeled as a piston (driver) placed in the center of such a plane wall so that the frontwave and backwave are separated by the plane wall. The effect of the baffle is to prevent front and backwaves from interfering with each other.
A finite baffle which extends around the driver over a minimum radius which is larger than the lowest wavelength of interest will have the practical effect of decoupling the frontwave and backwave. However, as the baffle radius around the driver becomes smaller in relation to a wavelength, interference between front and backwaves at that wavelength becomes greater.
An acoustic-suspension arrangement is sometimes referred to as an “infinite baffle,” because the backwave is effectively eliminated. The acoustic-suspension solution comes at the cost of increased power consumption to overcome air compression in the sealed box. An additional disadvantage of the sealed box is that increased thermal dissipation may be needed to offset heat caused by increased power consumption in the driver.
Background: Sound Image
Distance perception is a fairly difficult area of psychoacoustics: see Blauert at 116–137. However, in the case where the loudspeakers are focused to produce an intensity maximum somewhere outside the listener's head (e.g. in front of the listener), the listener will often identify the spatial location of the intensity maximum as the apparent source of the sound. In such cases the intensity maximum will define the apparent origin of a “sound image.” See Hartmann, “Sounds in space, sounds in your head, and sounds in between,” PROCEEDINGS OF 1997 WORKSHOP ON APPLICATIONS OF SIGNAL PROCESSING TO AUDIO AND ACOUSTICS 1 (1997); Komiyama et al., “Distance control of sound images by a two-dimensional loudspeaker array,” 13 J. ACOUSTICAL SOC. JAPAN (E) 171 (1992); both of which are hereby incorporated by reference. (Such a sound image can be thought of as a very simple case of acoustic holography.)
One of the more subtle psychoacoustic criteria for sound reproduction arises from the existence of such sound images. If a sound image location is too close to the listener's position, many listeners will feel a psychological discomfort which is analogous to the discomfort felt when another person talks too close to one's face. Generally, a preferred location for the sound image relative to the listener is approximately 12–24 inches in front of the listener's face, with 18–24 inches being more desirable. A possible exception to this “comfortable distance” concept is with the use of headphones, where an “inside the head effect” may be desirable.
Background: Driver Design
Audio scientists try to optimize drivers for the environment (such as free-space or baffled) in which they will be used. Typical design choices are coil size, magnetic gap, heat sinking, cone size, cone shape, overhang, and throw length. A driver designed for free-space operation generally will be engineered for a longer throw than one designed for use in an acoustic suspension box. A longer throw generates a larger volume-velocity to make up for sound cancellation due to interference between the frontwave and backwave. The throw of an acoustic suspension driver, on the other hand, is restricted due to air compression in the sealed box.
One drawback of using a speaker designed for sealed enclosures in an acoustically leaky chassis is that the voice coil will move outside the magnetic field (in other words, the coil has more excursion than it was designed to accommodate because the expected backside pressure is not present). This excess excursion will result in harmonic distortion. Additionally, in comparison to speakers designed for free space operation, speakers designed for sealed boxes are generally 7 to 10 dB down when put in a leaky chassis.
Computer with Speaker in Acoustically Leaky Chassis
The present application discloses a desktop computer model which has a driver mounted in the back side of the chassis. The chassis is acoustically a very leaky box, so the driver is designed as if it were a nearly freestanding driver. (That is, the driver is given a larger cone area and/or longer throw than would otherwise be required for a given sound pressure level at a given low-frequency limit.) Acoustic leakage through the front of the chassis helps to move the acoustic image forward by moving the intensity maximum forward.
The treatment of the box as being acoustically leaky means that the airflow required for thermal management has not been restricted. Because the budget for internal volume is fairly large, the driver can be large. A large cone size combined with the use of a long-throw driver permits the volume-velocity of the driver to be increased, thus increasing sound power at low frequencies. Treatment of the box as very leaky also means that no user changes or custom configurations will significantly disturb the expected acoustic environment.
The rear-firing speaker helps to keep the acoustic image moved somewhat to the rear, i.e. out of the user's face, and also helps to improve spatial ambience by increasing the ratio of reflected to direct sound. However, front-side leakiness of the box provides some acoustic emission toward the front, which is also advantageous.
Thus the present application teaches that the driver is preferably designed for operation in “free-space.” This is a different direction of improvement than has been conventionally followed: previous attempts used a driver mounted within the computer while treating the computer box, or a sub-box inside the computer chassis, as a loudspeaker box. Instead, the present invention tolerates or even increases the acoustic leakiness of the computer chassis.
The present invention is most advantageous in nonportable small computer systems (e.g. desktop or minitower systems). Power efficiency and volume are less tightly constrained in desktop systems than in portable computers. An audio system that does not interfere with modularity, ease of chassis assembly, and ease of maintenance is an advantage in desktop systems.
A particular advantage of an acoustically leaky chassis with a back-mounted driver is that the acoustic transmission through the front of the computer will provide a deeper soundstage and more of an impression of depth to the user.
In an alternate (perhaps superior) embodiment, an audio system is comprised of an acoustically leaky computer chassis, an equalizer, electrical gain staging, and a driver designed for nearly free-space operation. The equalizer may have predetermined equalization stages designed to work with a free-space driver. Of course, as discussed above, the equalizer and electrical gain staging are not necessary to practice the invention in the preferred embodiment.